api: unify Event and Link access patterns

[jtescher]

This makes the data in span Events and Links pub, but guards against future additions by making the structs non_exhaustive.

[codecov]

Codecov Report

Merging #757 (9977aa6) into main (027c1f8) will decrease coverage by 0.01%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##             main     #757      +/-   ##
==========================================
- Coverage   70.22%   70.21%   -0.02%     
==========================================
  Files         108      108              
  Lines        8804     8795       -9     
==========================================
- Hits         6183     6175       -8     
+ Misses       2621     2620       -1     

Impacted Files	Coverage Δ
opentelemetry-jaeger/src/exporter/mod.rs	59.76% <0.00%> (ø)
opentelemetry-api/src/trace/mod.rs	78.72% <100.00%> (+1.93%)	⬆️
opentelemetry-api/src/trace/tracer.rs	61.03% <100.00%> (ø)
opentelemetry-sdk/src/trace/span.rs	93.65% <100.00%> (ø)
opentelemetry-http/src/lib.rs	7.01% <0.00%> (-5.27%)	⬇️
opentelemetry-sdk/src/metrics/controllers/push.rs	83.33% <0.00%> (+3.33%)	⬆️

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[djc]

I like it. Was there a particular motivation for doing this now?

[jtescher]

Trying to get all the api changes needed before 1.0 release